To reduce reliance on petroleum and coal based fuels, at least some utilities have turned to solar energy to replace, or augment, conventional electric power plants. In desert regions, where clear days predominate, the switch to solar energy is very desirable. Of several types of solar energy systems, the central solar receiver plant has proven to be a highly reliable and efficient producer of large commercial quantities of power. For example, central solar plants can produce 100 Mwe of power or more.
A solar central receiver plant uses a plurality of sun tracking mirrors called heliostats. The heliostats reflect and concentrate solar energy on to a central receiver. One such system is disclosed in co-pending U.S. application Ser. No. 09/879,363, Titled “Thermally Controlled Solar Facet With Heat Recovery”, filed Jun. 12, 2001. Central receiver solar plants typically include a tall tower that holds the central receiver aloft to increase a field of view in order to allow many heliostats to focus energy upon the central receiver. A heat transfer fluid, e.g., molten salt, water, liquid metal, or air, flows through the central receiver absorbing the heat from the solar energy reflected by the heliostats. The heat is transferred through a turbine generator combination that utilizes the heat absorbed by the transfer fluid to create electric power.
In at least some conventional central receiver solar plants a preprogrammed controller controls the aiming of the heliostats. The controller continually predicts where the sun is and periodically positions the heliostat accordingly, e.g. every several seconds. Generally, the prediction is based on the date, time, longitude, latitude and elevation of the heliostat. Using the predicted sun location and the position of the receiver with respect to the heliostat, the controller calculates an azimuth and elevation angle for each heliostat. The azimuth and elevation angle are calculated so that each heliostat is position to reflect the sun light directly onto the receiver.
However, this aiming strategy is often inaccurate. Inaccuracies in the positioning of the heliostats results in efficiency losses of the central receiver solar plant. More specifically, if a heliostat fails to reflect the sun light directly on the receiver, a portion of the energy associated with the light will be lost. This type of energy loss is often referred to as spillage. Spillage reduces the efficiency of the system and often requires additional heliostats to compensate for the loss, which in turn adds significant plant costs. Additionally, the aiming inaccuracies can result in thermal damage to structures and devices near the receiver, which will also significantly increase plant costs. Furthermore, errors in the devices that physically adjust the position of each heliostat, for example, gear backlash and encoder errors can contribute significantly to heliostat positioning inaccuracies.
Generally, the heliostat fields of central receiver solar plants comprise between thirty and forty percent of the total capital investment needed for the overall solar plant. Therefore, increasing the accuracy and efficiency of a solar plant by increasing the number of heliostats or employing more expensive aiming device significantly increases plant costs.
Thus, a need exists to improve the methods and systems associated with positioning the heliostats in central receiver solar plants.